The present invention relates to a differential amplification circuit.
We previously developed a differential amplification circuit as shown in FIG. 3. This circuit comprises differential transistors Q10 and Q11, a constant current circuit CS1 provided at a common emitter of these transistors to flow a constant current 2Io, a high-precision current mirror circuit 1 provided at a collector of one transistor Q11 and a constant current circuit CS2 mounted on the output side of the current mirror circuit, where the constant current value is set at 1/2 of the value of the above-mentioned constant current 2Io.
This circuit puts out the difference (I.sub.1 '-Io) between the output current I.sub.1 ' from the high-precision current mirror circuit 1 and the current Io flowing in the constant current circuit CS2, and the circuit is arranged so that in the state where the differential transistors are well-balanced with each other, this current difference shall be maintained at zero. By this arrangement, it is made possible to use a region of a good linearity of the transmission characteristic in the differential amplification circuit.
In the state where input voltages Vin and Vref applied to the bases of the differential transistors Q10 and Q11 are well-balanced with each other, each of the emitter currents of the transistors Q10 and Q11 is Io. Accordingly, supposing that the base current of the transistor Q11 is expressed as Ib, I.sub.1 is equal to (Io-Ib), and in the high-precision current mirror circuit 1, the ratio of the input current I.sub.1 to the output current I.sub.1 ' becomes equal to substantially 1. Therefore, the output current I.sub.1 ' of the current mirror circuit 1 is substantially equal to (Io-Ib).
Since the constant current circuit CS2 as the output circuit is for flowing the current Io, in the input-balanced state an offset output current of -Ib is generated in the output current.
We made investigations on a method of causing offset in emitter bias current values of the input differential transistors for reducing the offset output current to zero.
However, since production of a difference in bias currents results in a change of the temperature characteristic dVbe/dT of the base-emitter voltages Vbe in the forward direction of the transistors, the output of this differential amplification circuit is changed according to the temperature change. Furthermore, the above base current Ib per se is changed depending on the temperature because of the temperature characteristic of the common emitter current amplification rate hfe. Thus, it has been found that the offset quantity per se has a temperature dependency or temperature characteristic and the output is changeable depending on the temperature change and is hence unstable.